Equine Health, Internal Parasites and Deworming

Equine Deworming: Horse Intestinal Parasites

What are the most common internal parasites in horses? How do you control intestinal parasites in horses? Intestinal worms cause damage and inflammation to a horse’s gut lining and intestinal tract. Parasites can also lead to colic. Intestinal endoparasites can be seen in various stages in the horse; they can be eggs, larvae, and as adults that produce eggs. That’s why it’s important to know what parasites you need to protect your horse against:

Threadworms in Horses

After a mare gives birth, larval stages of threadworms within the mares tissue activate and move to the mammary glands and into her milk. As her foal suckles, these threadworm larvae move into the foal’s intestinal tract. Some farms will deworm the mare with Ivermectin in an attempt to reduce these larvae, but it is not clear if this is effective – there is no label approval for any dewormer to treat larval stages of threadworm in a broodmare. Larval stages of threadworms are seen in milk on day five after the mare gives birth and stay in milk up to 50 to 60 days. There are no threadworms in colostrum.

Threadworms in Horses Symptoms

These threadworm larva from the milk will develop in the foal’s small intestine and become adults very quickly, in only 10 days! These adult female threadworms will burrow into mucosa, creating inflammation. This leads to the foal experiencing profound diarrhea, dehydration, weakness and electrolyte imbalance. Treating this diarrhea can be difficult. Providing a probiotic such as Probios® Equine One Gel, or a colostrum supplement such as Breeder’s Edge Nurture Mate for Foals, can help support a foals immune system and normal digestive function. Foals will get immunity to threadworms by six months of age so no more eggs will be seen after this. Foals with no previous exposure to strongyloides are still a primary challenge. Only previously challenged foals with exposure to the parasite get immunity.

Unfortunately you can’t see adult threadworms in fecal piles of foals in day to day life; however, a fecal test will show the eggs of this parasite.

How to Treat Threadworms in Horses

There are only two dewormers approved to clear threadworms. Ivermectin paste, such as Eqvalan Paste Generic, will kill these adult threadworms in foals. Ivermectin is approved in all ages of foals and kills the adults and the larvae. Many farms will treat foals with this prior to day 10 to reduce diarrhea chances. Oxibendazole has also been shown and used to kill the adult threadworms in foals. Many farms will treat foals with one of these at day 10 to 20 of life to try to reduce diarrhea and then come back in 2 months with the other option – to cover these parasites until immunity. There are no known reports of Ivermectin or Oxibendazole resistance in these parasites.

Roundworms (Ascarids) in Horses

Female roundworms can range from 8 to 15 inches long and they create huge amounts of eggs that will develop infective larvae. One female roundworm can lay 200,000 eggs a day! Roundworm eggs are very tough – they can survive in cold/heat up to 10 years in a field – so the foal’s environment may contain millions of possible ways for roundworms to get into your foal. Adult roundworms start to be seen in foals around two to three months of age.

Symptoms of Roundworms in Horses

Because so many roundworms can get into your foal, once they develop into adults, they take up huge amounts of space in the lumen of the small intestine which leads to impactions. If a horse is full of worms, food is not moving, so the horse experiences colic. Other signs of roundworms are weight loss, poor hair coat, potbellied appearance and gastric reflux.

When the foal ingests an infective egg with a larvae in it, the larvae will burrow into the gut wall, travel to the liver and then the lungs. About one week later the foal will develop a chronic cough. Larval stages in lungs can create chronic coughing. The roundworms will mature as a larvae for two weeks, then get coughed up, swallowed again and travel back to the small intestine where two months later they are adults producing eggs. Roundworm adults are seen at two to three months of age in a foal.

When a foal reaches five months of age they typically experience the peak number of adults roundworms. Over the next seven months, (by 1 year of age), most foals will become immune to roundworms. Unfortunately, not all horses become immune. Some adult horses may still have adult roundworms due to abnormal immunity, that’s why following a regular deworming schedule is recommended.

How to Treat Roundworms in Horses

Due to the roundworms terrible effects on foals, many roundworm dewormers are used multiple times in a foal’s life. Equimax contains both ivermectin and praziquantel and is effective at killing both threadworms and roundworms in foals four weeks of age and older. Safe-Guard Equine Dewormer contains fenbendazole which also helps control ascarids (roundworms). Both are great options for roundworm prevention and treatment.

The Oxibendazole or the Ivermectin used on the second deworming in a foal at two months old to get strongyloides will also get roundworms. An advantage to doing Ivermectin as the 2nd dewormer is that it gets roundworm larval stages in the lungs, so will reduce the chronic cough seen in larval round worms stages as it passes into lungs.

Tapeworms in Horses

Tapeworms are a major cause of colic in young and adult horses. These intestinal worms in horses are the size of a chiclet piece of gum, but they gather in a very bad place. They converge where the small and large intestines connect and the passage way here is only about five inches in diameter– so these “chiclets” gather on the periphery, and overtime create a smaller and smaller diameter for food and water to get through. From 4 inches to 3 inches to 2 inches – so impactions occur resulting in colic.

The tapeworms life cycle in a horse is six months. It starts with the foal/yearling/adult horse eating grass that harbors a small mite that gets eaten and goes to the connection site. These mite eats tapeworm eggs so when a horse eats a mite, the larvae in the mite moves to connection site.

Since the tapeworm lifecycle is six months, it’s critical to deworm for tapeworms two times a year, that is usually done in the spring and fall.

Strongyles in Horses

Strongyle larvae live on dew drops of grass. One drop of dew can hold 1,000 larvae. As a horse eats larvae the larvae migrate to the large intestine and develop into adults. These create terrible damage to the gut lining. Small strongyles are thought to be one of the biggest causes of colic in horses.

Thanks to Ivermectin over the last 20 years, large strongyles have gone from the #1 cause of colic in horses to almost wiped out. Ivermectin kills all adult and all larvae of large strongyles. Their eggs on fecal tests are tough to tell from small strongyles. Larval migration attack blood vessels in the lower extremities and adults in intestine create lining damage and blood loss. Large strongyles are about 1 inch long and thankfully almost all dewormers kill the adults.

Small and large strongyles have an approximate six month life cycle. They migrate larvae in winter and most lay eggs in spring. Encysted small strongyles in the horses gut lining peak in winter, often leading to colic in horses in winter due to this damage. Like tapeworms, you need to treat strongyles two times per year since both tapeworms and strongyles have a six month cycle. Due to small strongyles encysted larvae not killed by Ivermectin and peak of these in winter, you need treat these in winter. Moxidectin products and a double dose of Panacur (fenbendazole) for five days in a row kill these.

Equine Bots (Gasterophius larvae)

Your fall deworming schedule needs to address bots. Their number peaks in full in the stomach. Bots are NOT worms. They are a fly larvae. Owners often see bot eggs on hairs of a horse’s legs and use a bot knife to remove these. Bot eggs on the horse’s leg hair create an itch. Horses then lick their leg hair to itch, the eggs go in their mouth, the larvae in the egg goes into the horse’s mouth mucosa (lips and gums), emerge out and go to the horses stomach. These bots create a huge problem in a horse’s stomach. A horse can have dozens of bots in its stomach, amounting to the size of a honeybee. That leads to a disturbance in the horse’s digestion. The bots screw into the stomach lining creating nasty wounds.

Bots stay in the horses stomach for 8 to 12 months, then release, pass out in manure one to two months later then turn into adult flies called warble flies or gadflies. They are not stable flies. Adult bots look like small wasps and buzz loudly, making a horse nervous.

Equine Bot Flies Treatment

Ivermectin and Moxidectin (QuestPlus) clear bots. Since the number of bots peaks in late fall as the fly season is ending, the use of an Ivermectin/Praziquantel dewormer such as EquiMax in the fall to get tapeworms will also get bots.

Equine Pinworms (Oxyuris)

Female pinworms lay sticky eggs on the outside of the horse. They wiggle from the large intestine to the anus, and apply these eggs there. The eggs are going to create an itch, so horses rub on posts/barn walls/buckets, depositing eggs all over the place. The tail rubbing due to itch often results in broken hairs at the base of the tail (this sign is also common in summer eczema – so getting deworming history can help in determining which one is the problem.)

Horses eat these eggs and the larvae travel to the large intestine, become adults. Due to the fact that the eggs are not in manure directly (maybe after fall out in manure), pinworm eggs are not seen in fecal tests usually. So we can’t say “I see no pinworms eggs in fecal test so not pinworms”

All equine dewormers used in horses today kill pinworms. This has greatly cut down on the environmental load and also the self-mutilation of tail/skin of tail from head rubbing.

Habronema in Horses

Habronema in horses is more common in warmer climate areas such as the southern United States; (Florida, the Carolinas, Alabama, etc.). They are the true stomach worm of horses since that is where the adult parasite lives. Adult Habronema produce eggs in the stomach and are in manure but NOT usually seen in standard fecal tests. They are very hard to see in a fecal test, so again, another reason why you can’t rely on a fecal test to address Habronema.

The Habronema eggs in the environment are eaten by stable or horse flies. The larvae is now inside that fly now and those flies deposit these “passengers” around horses mouth. When the horse licks the larvae travel to the stomach and develop to adults in two months.

If the flies deposit larvae on the horse’s legs/body instead of the mouth, these larval passengers burrow into the skin creating nasty “summer sores”. Flies can also deposit passengers into the horse’s eyes.

Use Ivermectin after surgery to clean up the summer sores is effective and kills these misdirected passengers. Ivermectin can be placed into the wound. To kill adults of Habromena in the horse’s stomach, Moxidectin dewormer products such as QuestPlus are the best choice. Ivermectin has label approval for this also, but at times is not as effective as Moxidectin in some studies.

Are Horse Fecal Tests Accurate?

Horse fecal parasites tests give you information, but it’s not the whole picture. Many of these parasites never show up on a fecal test. Parasites are missed in standard fecals for several reasons such as:

  • Bots in stomach is a larvae and produces no eggs.
  • Pinworm females deposit eggs outside of the horse around the anus/tail; therefore, no eggs in fecal.
  • Tapeworms are a huge colic generator. Sections of these tapeworms break off with eggs in the sections, so only a few eggs get scattered in feces.
  • The larval stages of worms migrating into your horse’s organs have no eggs.
    1. Encysted strongyles in lining of gut. (small strongyle)
    2. Migrating larva in roundworms in liver/lung
    3. Migrating larvae in large strongyles. In blood vessels.
  • Habronema adults in stomach fecal tests miss the eggs they produce in feces.

That’s why deworming needs to be done on a set schedule throughout the life of the horse.

Fecal Egg Count in Horses

Equine fecal tests pick up strongyle eggs in yearlings (large and small strongyle eggs) but usually no other parasites eggs. So fecal tests in older yearlings and adult horses are not fecal tests in reality – they are strongyle tests. It is good to have less than 200 eggs/gram on tests. If have a large herd, usually test 20% of herd to get an idea of status.

As far as horses passing these worms to humans, the good news is all intestinal endoparasites of young and adult horses have no risk to people contracting them. For example, you can’t get horse roundworm, horse tapeworm or horse pinworms. If you have further questions on deworming a horse, foal or need help finding the best equine dewormer, give us a call at 800.786.4751.

Equine deworming chart

Written by: Frank Reilly, DVM

Senior Doctor at Equine Medical & Surgical Associates

Frank Reilly, DVM has been in equine practice for 35+ years and has worked on six world-record racehorses. He is the Senior Doctor at Equine Medical & Surgical Associates, Inc. and a member of the AVMA, AAEP, NAEP, and IAPF. Dr. Reilly specializes in Equine Cushings, Insulin problems, Summer Eczema, Vitamin E deficiencies, COPD/Asthma, and Horse Foot Canker. Dr. Reilly is a 35+ year member of American Veterinary Medical Assoc. and the American Assoc. of Equine Practitioners. He is also a member of Pennsylvania Veterinary Association, the Northeast Assoc. of Equine Practitioners and is on the Board of Directors and Treasurer of Ryerss Farm-America’s First Horse Retirement and Rescue, established in 1888.

If you need help, call us at 800.786.4751.